Many applications depend on the ability to read or sense information at very high resolution. For example, in a storage system, such as a CD (compact disc) reader, a focused laser beam is used to read information patterns on a disc. However, since such conventional optics is based on refraction and focusing of electromagnetic radiation, it comes with a fundamental constraint in spatial resolution. Specifically, the propagation of electromagnetic radiation over distances larger than the optical wavelength (λ) acts as a filter of finite spatial bandwidth, which results in the familiar diffraction limited resolution of ≈λ/2. For example, for λ=1 μm, the maximum possible spatial resolution is ≈0.5 μm, which is far from adequate in many now-known and future applications.
As a solution to this resolution problem, near-field techniques have recently been introduced by utilizing non-propagating “near-fields” (D. W. Pohl et al., Appl. Phys. Lett. 44, 651 (1984); A. Lewis et al., Ultramicroscopy 13, 227 (1984)). Due to the lack of propagation, such fields do not obey the diffraction limit (M. A. Paesler, P. J. Moyer, Near-field Optics (John Wiley & Sons, New York, 1996)). Generally speaking, in order to generate such near-fields, (i) an incident driving field with the wavelength λi and (ii) an object (e.g., a subwavelength aperture, a sharp object tip, or a sharp edge) with much higher (spatial) wavelengths λo (λo<<λi) is needed. Such an arrangement will be referred to herein as a “near-field source,” which can “focus” or “concentrate” electromagnetic radiation far below the diffraction limit. This near-field source can then be used to excite another object (typically a sample), which will be referred to herein as a “near-field receiver.” The response of the near-field receiver, due to the excitation of the near-field source, results in the generation of propagating waves (e.g., due to scattering, absorption, extinction, fluorescence, chemiluminescence etc.), which can then be monitored in the far-field by some conventional detector setup.
FIGS. 1-3, in conjunction with the following explanation, give examples of how such near-field sources have been realized. Referring first to FIG. 1, most near-field sources utilize a subwavelength aperture 100, which is placed in a propagating wave 102. In most cases, the propagating wave 102 is a focused laser beam (e.g., as is shown in U.S. Pat. No. 4,604,520). In this example, a small fraction of the incoming field 102 is converted into a non-propagating near-field 104, which “leaks” out of the aperture 100 and can be used to excite a sample/work piece (not shown).
In an alternative approach, as shown in FIG. 2, which can offer substantially higher resolution and stronger near-fields, a sharp object tip 200 is driven externally by an electromagnetic laser field 202 in order to generate a highly localized near-field source 204 (e.g., U.S. Pat. No. 4,947,034). In some cases, antenna effects are exploited to further enhance the strength of the near-field (e.g., U.S. Pat. No. 6,771,445; see also “Strength of the electric field in apertureless near-field microscopy” Y. C. Martin, H. F. Hamann, H. K. Wickramasinghe, J. Appl. Phys. 89, 5774 (2001))
As a third example, FIG. 3 shows a driving field 302 that is reflected via internal reflection at a surface 304 of a prism 300 (e.g., as is shown in U.S. Pat. No. 5,018,865). On the outside of the prism 300, due to the abrupt change at the prism-air interface 304, a “one-dimensional” electromagnetic near-field 306 is generated, which decays exponentially away from the surface 304, but which is still diffraction limited in the lateral dimensions.
Unfortunately, the arrangements of FIGS. 1-3 in addition to generating a localized non-propagating near-field, also scatter some of the driving field into the far-field. As a result, some fraction of the driving field directly hits the detector. Such signals are referred to as “background” and are shown as elements 106, 206, and 308 in FIGS. 1-3, respectively.
The usefulness or quality of a near-field source is largely determined by the ratio of near-field versus far-field (background) signal strengths. All traditionally-used near-field sources (FIGS. 1-3) for reading information have in common that the driving field is generated by a laser or other light source. As a result, the various methods for generating near-fields are accompanied by several difficulties and challenges. Some of the near-field sources show low near-field strengths (FIG. 1) and low confinement (FIGS. 2 and 3). Others, such as that shown in FIG. 2, although providing strong near-fields and very high confinement, can generate fairly large propagating background signals at the same wavelength as the near field energy due to the driving field 202, which can somewhat obscure the response of the near-field receiver. In addition, the strength of near-field of these configurations is very sensitive to the polarization, the wavelengths, and the focus of the driving field, which further complicates the control of such near-field sources.
Therefore a need exists to overcome the problems with the prior art as discussed above.